The equation of a circle $C$ is $x^2+y^2+2x-35 = 0$. What is its center $(h, k)$ and its radius $r$ ?
Explanation: To find the equation in standard form, complete the square. $(x^2+2x) + (y^2) = 35$ $(x^2+2x+1) + (y^2) = 35 + 1 + 0$ $(x+1)^{2} + y^2 = 36 = 6^2$ Thus, $(h, k) = (-1, 0)$ and $r = 6$.